The present invention concerns a fluid pressure control device for controlling at least one working chamber of a servo motor, the device having one or more control valves which are operated via operating means through relative movements between a power input member and a power output member. The power output member is connected with the power input member by means of flexible coupling elements, whereby the servo motor is in operative connection with the power output member. The control valves are provided with reaction areas for signalling developing pressures at the power input member, and/or in operative connection with one or more corresponding reaction members.
Pressure control devices are required in all cases where it is necessary to increase power or torsional moments. Common fields of application are servo-steering systems of motor vehicles or reinforcement for rudder mechanisms on ships. Since some of the problems which occur during practical application are more easily explained by referring to a servo-steering, such a device will be referred to in the following description, without, however, any intention to limit the invention to use with servo-steering devices.
In a servo-steering device, the power input member is connected with a steering wheel, while the power output member is operatively connected to steerable wheels, whereby the servo motor assists the movements of the steering wheel in the sense of an amplifier. Numerous publications have already dealt with servo steering devices, with particular regard to the arrangement of the control valves, as well as for producing relative movement, many proposals have been made, while other proposals are concerned with the arrangement and construction of reaction members.
In this regard, the arrangement and construction of coupling members have been the subjects of publications such as, for example, German Pat. Nos. 936,973; 1,003,610; 1,113,642; and 1,137,963, as well as the German Disclosure published for Inspection, No. 1,924,032. The coupling members may be provided with an initial resistance so that relative movements result only after the initial resistance has been overcome. It is, however, also possible to construct a coupling without initial stress. Depending on the construction, it is, therefore, possible at first to steer without amplification or power assistance or only with low amplification. Little or no amplification is desired, for example, in fast motor vehicle travel and/or during icy road conditions. However, since during purely mechanical power-transmission, the servo motor also performs a movement and therefore must either distribute or absorb pressure medium, the pressure medium control devices of the above-described prior art type have so far always been provided with control-valves of an "open-center contruction", i.e., in a neutral position of the control valves, each working area of the servo motor is connected with at least one outlet so that a free movement is present. Thus, it is also possible to connect all ports with each other so that there results a more or less positively throttled circulation of the pressure medium.